SPP1 and cancer: OPN is a phosphorylated glycoprotein that bridges cell and matrix inorganic substances and is closely associated with the occurrence, progression and metastasis of malignant tumors (Le et al., 2003; Vergis et al., 2008; Raja et al., 2014; Ostheimer et al., 2017; Cao et al., 2019; Nishio et al., 2021).